Just getting started on home brewing and I used boiled water in my airlock. I've seen that some places recommend using sanitizer as the airlock "fill". Is it ok to use straight up StarSan in there or should I dilute with water?

Allow airflow only in the direction of lower pressure. This qualifies pretty much any liquid.

Not have funk grow in it. This means something you could use to sanitize things, or anything with high alcohol content (eg, I use vodka sometimes).

Not hurt your beer if it ends up getting sucked back down into the fermenter. This means no bleach or other rinse-required sanitizers, and no undiluted no-rinse sanitizers (since they cease to be no-rinse in high-concentrations).

So:
You can starsan, but at in the same level of dilution recommended on the bottle. I usually just save a cup from when I sanitize my equipment on brewday and use a bit of that.